The current trend in connector design for connectors used in the computer field is to provide both high-density and high-reliability connections between various circuit devices that form important parts of the computer. High reliability for such connections is essential due to potential end product failure, should vital misconnections of these devices occur. Typically, integrated circuit chips are attached to a chip carrier, thermally conductive module chip carrier, circuit card, or board by solder bonding, brazing, controlled collapse chip connect, wire lead bonding, metal bump bonding, tape automated bonding, or the like.
One prior technique for providing various interconnections is referred to as a wire bond technique, which involves the mechanical and thermal compression of a soft metal wire (e.g., gold) between one circuit and another. Such bonding does not lend itself readily however, to high-density connections because of possible wire breakage and accompanying mechanical difficulty in wire handling.
Another technique involves strategic placement of solder balls or the like between respective circuit elements (e.g., pads) and reflowing the solder to effect interconnection. Although it has proven extremely successful in providing high-density interconnections for various structures, this technique does not prevent or decrease destructive forces from propagating in the Z-direction.
In yet another technique, an elastomer has been used which includes a plurality of conductive paths (e.g., small diameter wires or columns of conductive material) to provide the necessary interconnections. Known techniques using such material typically possess the following deficiencies: (1) high force necessary per contact which can be inherent in a particular design and exacerbated due to non-planarity of the mating surfaces; (2) relatively high electrical resistance through the interconnection between the associated circuit elements (e.g., pads); (3) sensitivity to dust, debris, and other environmental elements that could adversely affect a sound connection; and (4) limited density (e.g., due to physical limitations of particular connector designs).
When connecting the surfaces of two components, such as a ceramic material module and a glass-epoxy printed circuit board, a significant amount of compliance is required for the glass-epoxy bond. This compliance must be accommodated by the connector, which must overcome the flatness and irregularities inherent in the surfaces of the board and the module as well as their different thermal coefficients of expansion. The planarity and rigidity of the ceramic is relatively good. As pressure is applied to the edge of the ceramic component to connect a plurality of connectors, the glass-epoxy printed circuit board has a tendency to bow as the area array increases. Moreover, during use of the completed device, heating causes uneven expansion of the board and the module, thereby leading to further bowing. This bowing must be accommodated by the connector. It should be noted that, as the body size of the modules gets larger, mismatch due to differing coefficients of thermal expansion increases. For example, as the size increases to greater than about 32 mm (1.25 inches), the mismatch becomes great.
Area array packages or ball grid array (BGA) modules, including such modules as ceramic ball grid array (CBGA) modules and tape ball grid array (TBGA) modules, and land grid array (LGA) modules, typically have 90/10 weight percent lead/tin (Pb/Sn) solder balls on the underside of the package. These solder balls are connected to adhesion pads, typically copper (Cu) pads, residing on a printed circuit board (PCB) by reflowing 63/37 Pb/Sn eutectic solder paste. Interconnectors or interposers have been used between the solder balls and the adhesion pads to facilitate adhesion.
Currently, most of the solder ball connector (SBC) and solder column connector (SCC) interposers available in industry require mounting holes to be placed into the card or board. The mounting holes detract from wirability and increase the footprint of the card or board because the holes are defined as "keep out" zones for surface components.
An SBC or SCC interposer should be able to be used with a card or board designed for a surface-mounted module. This possibility gives the board designers the flexibility to design a board that can be used directly with surface-mounted modules or with interposer connectors. Presently available interposers that are dendrite plated (e.g., the Flexiposer.TM. interposer available from International Business Machines Corp. of Armonk, N.Y.) do not have adequate compliance to compensate for the forces that affect the Z-axis forces of a stacked connection between a module and a board or card.
A need exists, therefore, for an interposer having compliance and flexibility to be used in different packages depending on the application. Moreover, a need exists for a surface-mounted interposer that can be used with an existing card or board that was originally designed for direct solder of an SBC/SCC module. This type of connector, if compliant, would be a great advantage over current connectors.
Although the art of circuit module to supporting substrate connections is well developed, there remain some problems inherent in this technology. One particular problem is the bowing and formation of a crack in the Z-direction caused by compression forces and differing thermal coefficients of expansion. Therefore, a need exists for a structure that increases the reliability of the connection between a circuit module and a supporting board or card.